Helped by a big edge in ad spending and a growing acceptance among GOP voters, Mitt Romney leads Rick Santorum 39% to 31% in the Wisconsin Primary, according to a new Marquette Law School poll.

That’s a major shift since Marquette’s last survey in February, which showed Santorum ahead by double digits. Ron Paul is running third in the poll with 11%, followed by Newt Gingrich with 5%.

Romney not only vaulted past Santorum among GOP primary voters, but he gained on President Barack Obama as well.

Obama leads Romney 48% to 43% among all Wisconsin voters in a fall match-up. But that’s a much smaller lead than Obama enjoyed in Marquette’s February survey, when the President led Romney 53% to 38%.

Underlying these changes is an improvement in Romney’s image in Wisconsin since last month, when Marquette’s poll coincided with a low point in Romney’s fortunes following a February losing streak.

In that February poll, almost twice as many Wisconsin voters viewed Romney unfavorably as viewed him favorably.

In the latest poll, his numbers are still more negative than positive with the Wisconsin electorate as a whole. But they have gotten better:

More relevant for the April 3 primary, Romney’s numbers have improved among GOP voters. The chart below shows the three-month trend in Marquette’s polls among Republican voters and voters who lean Republican:

Romney’s “positives” are up among Republicans since January and February – and his “negatives” are down.

Santorum’s standing among Wisconsin Republicans is still quite positive, but his negatives have gone up since February:

The ad wars appear to be playing a role in all this.

As of Sunday when the new poll was completed, Romney and a pro-Romney PAC had spent almost $2 million airing TV ads in Wisconsin, according to a media buyer who tracks political advertising. (That figure doesn’t include ads that hadn’t aired yet). Santorum had spent under $100,000.

The rise in the share of Republicans who view Santorum negatively (from 10% to 23%) may be a product of that ad blitz. The Restore Our Future PAC had aired about $1.2 million worth of ads as of last Sunday attacking Santorum over his record on earmarks and federal spending. The Romney campaign has also used robo-calls to bash Santorum on earmarks.

But Santorum’s personal popularity with GOP voters here remains positive. His favorability rating among GOP voters and voters who lean Republican is virtually the same as Romney’s in the new Wisconsin poll. Compared with February, his share of the GOP primary vote has dropped a mere three points, from 34% to 31%.

The big change has been in Romney’s support, which more than doubled:

That suggests the attacks on Santorum aren’t the only reason that Romney has overtaken him here. Also at work may be an uptick in Romney’s image and what many GOP insiders here see as a pivot toward Romney among Republican voters who view him as the all-but-certain nominee. Romney has used his own advertising in Wisconsin to tout his credentials as a fiscal conservative.

“It’s not like (Santorum’s) support has collapsed from where it was in February,” said political scientist Charles Franklin, who did the Marquette poll. “The huge difference was Romney going from 18% to 39%.”

Romney spokesman Ryan Williams said the new poll shows that “Wisconsin voters are clearly rallying around Mitt Romney’s pro-jobs message,” and view him as “the only Republican candidate who can defeat President Obama and undo the damage caused by his failed agenda and disastrous fiscal policies.”

Asked about the poll while campaigning in Wisconsin on Tuesday, Santorum said his time investment in the state would pay off (he has campaigned three days here and counting, while Romney is yet to arrive). And he noted that a Rasmussen poll last week had Romney up by a bigger margin -- 13 points.

"It’s better than 13. It’s eight (points down). That’s a movement in the right direction,” said Santorum. “I’ve seen this happen in other states where we’ve been down. And in spite of the overwhelming money being spent against us, we were able to claw our way back. We just work the state. We’re working all day today. We’re working most of the day tomorrow. We’ll be back Friday, Saturday, Sunday and Monday."

The Wisconsin Voter is a blog about elections, political trends and public opinion in Wisconsin and the Upper Midwest. It is less about politicians than the people who elect them. It’s aimed at political junkies and general readers alike. Its subjects include:

The role this state and region play as electoral battlegrounds.

Voting patterns and trends at the local, state and regional level.

What makes voters here different from voters in other places.

Public opinion and the election climate.

Craig Gilbert is the Journal Sentinel's Washington, D.C. Bureau Chief and national political reporter.

Charting how each of Wisconsin’s 72 counties has trended politically compared to the U.S. as a whole over 60 years of presidential voting. Use the pull down menu to see charts for individual counties. Click here for an explanation of how the charts were done and how to read them.